1. Field of the Invention
The present invention relates to signal processing in a wireless communications system, and, in particular, to channel estimation using blockwise analytic matrix inversion in a wireless communication system.
2. Description of the Related Art
The Universal Mobile Telecommunications System (UMTS) is a high-speed cellular radio system that provides digital data and voice communications. UMTS has recently evolved from 3G systems to 3.5G systems using High-Speed Downlink Packet Access (HSDPA) and High-Speed Uplink Packet Access (HSUPA), and still continues to evolve. The UMTS Long Term Evolution (LTE) protocol is currently being specified in 3GPP Release 8 to ensure its competitiveness for the next ten years and beyond. LTE, which is also known as Evolved UMTS Terrestrial Radio Access (UTRA) and Evolved UMTS Terrestrial Radio Access Network (UTRAN), provides new physical-layer concepts and protocol architectures for UMTS. See, e.g., Application Note 1MA111, “UMTS Long Term Evolution (LTE) Technology Introduction,” Rohde & Schwarz GmbH & Co. KG, available at http://www2.rohde-schwarz.com/en/service_and_support/Downloads/Application_Notes/, hereby incorporated by reference in its entirety.
According to the 3GPP Release 8 standard, the LTE downlink uses Orthogonal Frequency-Division Multiple Access (OFDMA) modulation. The LTE uplink uses Single-Carrier FDMA (SC-FDMA) modulation, which allows a relatively low-complexity receiver implementation in the base station. See, e.g., “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8),” 3rd Generation Partnership Project, 3GPP TS 36.211 V8.7.0 (May 2009), hereby incorporated by reference in its entirety.
FIG. 1 depicts a simplified block diagram of an LTE uplink 100, including an LTE uplink transmitter 110 (e.g., located in a UMTS mobile terminal) and an LTE uplink receiver 130 (e.g., located in a UMTS base station). In transmitter 110, serial data 112 to be transmitted is modulated (via quadrature-amplitude-modulation (QAM) or quadrature-phase-shift-keying (QPSK) modulation) by modulator 114 and converted into N parallel streams by serial-to-parallel converter 116. The N parallel streams are then input to an N-point discrete Fourier transform (DFT) unit 118 (or, alternatively, a fast Fourier transform (FFT) unit) to convert the streams to the frequency domain. Subcarrier mapping unit 120 maps the N frequency-domain streams to M available subcarrier frequencies (where M>N) and adds a plurality of demodulation reference symbols (DMRS) symbols (referred to below as pilot signals) in the frequency-domain (as regular OFDM symbols). Notably, the OFDM-based pilot signals and the SC-FDMA-based data symbols are separated in time. M-point inverse discrete Fourier transform (IDFT) unit 122 (or, alternatively, an inverse fast Fourier transform (IFFT) unit) then uses the frequency-domain, subcarrier-mapped streams as bins to create a block of M SC-FDMA symbols, each mapped to a different subcarrier frequency. The M SC-FDMA symbols output from IDFT unit 122 are serialized by parallel-to-serial converter 124 to create a time-domain SC-FDMA signal, and cyclic extension unit 126 adds to the time-domain SC-FDMA signal a cyclic prefix that enables frequency-domain processing at receiver 130 and thus facilitates significantly a reduction in the overall computational complexity of receiver 130. Finally, DAC/RF unit 128 converts the time-domain signal to analog and transmits the analog signal over the LTE uplink.
In receiver 130, RF/ADC unit 132 receives the transmitted analog signal and converts it to a digital signal, and cyclic extension unit 134 removes the cyclic prefix. The resulting signal is converted into M parallel SC-FDMA symbols at serial-to-parallel converter 136, and the M parallel SC-FDMA symbols are input to M-point DFT unit 138, which removes the M subcarrier frequencies and outputs M parallel words. The M parallel words are then input to (i) channel estimator unit 150 and (ii) subcarrier de-mapping/equalization unit 140. Channel estimator unit outputs, for each parallel word, an estimated channel transfer function, based on a locally generated pilot signal and a corresponding pilot signal contained within the received signal. Subcarrier de-mapping/equalization unit 140 (i) equalizes the data for each sub-carrier in the frequency domain based on the corresponding estimated channel transfer function from channel estimator unit 150 and (ii) de-maps the M parallel words to N parallel words. N-point IDFT unit 142 then converts the N parallel words to N time-domain parallel signals, and parallel-to-serial converter 138 converts the N time-domain parallel signals to serial data. The serial data is then demodulated by detector/demodulator unit 146 to recover data 148.
To assist with channel estimation, the LTE uplink protocol includes the transmission of one or more known pilot signals at regular intervals, along with data signals. As described above, channel estimator 150 in receiver 130 uses these transmitted pilot signals to estimate channel characteristics in the LTE uplink. Equalizer unit 142 in receiver 130 then uses the channel estimates to enable accurate data reception and demodulation. Conventional techniques for channel estimation include, e.g., the linear Minimum-Mean-Square-Error (LMMSE) and Least-Squares (LS) techniques.
An LTE uplink may also include advanced antenna technologies, such as Multiple Input Multiple Output (MIMO). See, e.g., A. Toskala et al., “Utran Long Term Evolution in 3GPP,” IEEE 17th International Symposium on Personal, Indoor and Mobile Radio Communications, pp. 1-5, September 2006, hereby incorporated by reference in its entirety. In a MIMO-based system, there are at least two transmitter antennas (each one corresponding to separate MIMO user) and at least two receiver antennas. (In a MIMO-based system, the number of receiver antennas is conventionally greater or equal to the number of transmitter antennas). Further, more than two transmitter antennas and two receiver antennas (e.g., four transmitter antennas and four receiver antennas (4×4 MIMO) may be used. Indeed, the 3GPP Release 8 standard specifies tests of the Physical Uplink Shared Channel not only with two receive antennas but also with four receive antennas. See, e.g., “Evolved Universal Terrestrial Radio Access (E-UTRA); Base Station (BS) Radio Transmission and Reception,” 3rd Generation Partnership Project, 3GPP TS 36.104 V8.5.0 (March 2009), hereby incorporated by reference in its entirety. One may therefore assume that the number of MIMO transmitting antennas is less or equal to two and four, respectively.
FIG. 2 represents a simple two-by-two MIMO LTE system 200 having two transmitter antennas 202 and 204, which transmit signals over four transmission paths 206, 208, 210, and 212 to two receiver antennas 214 and 216, where each receiver antenna 214, 216 receives a signal corresponding to the superposition of signals arriving over two different transmission paths. For example, the signal received at receiver antenna 214 corresponds to the superposition of (i) the signal transmitted from transmitter antenna 202 via transmission path 206 and (ii) the signal transmitted from transmitter antenna 204 via transmission path 208. In general, each transmitter antenna 202, 204 is associated with its own transmitter analogous to transmitter 110 of FIG. 1, and each receiver antenna 214, 216 is associated with its own receiver analogous to receiver 130 of FIG. 1.
In order to separate the transmitted pilot signals from each MIMO transmitter antenna 202, 204 received at each receiver antenna 214, 216, MIMO LTE system 200 may employ cyclic-shift transmit diversity (CSTD). CSTD is an adaptation of the idea of delay diversity to OFDM and SC-FDMA systems. With CSTD, each antenna element in a transmit array sends a circularly shifted version of the same pilot symbol. See, e.g., Javvin Technologies, Inc., Wireless Technology Terms, Glossary and Dictionary, at http://www.javvin.com/wireless/CSTD.html. For example, constant-amplitude zero-autocorrelation (CAZAC) sequences may be employed to provide a CSTD transmission scheme. Thus, in MIMO LTE system 200, the signal transmitted by transmitter antenna 204 is a circularly shifted version of the signal transmitted by transmitter antenna 202.